The recent recognition that cellular homologs of the avian retrovirus v- erbA code for thyroid hormone receptors and are members of a superfamily of regulatory genes has opened many new areas of investigation. The findings are at least 2 genes and 3 mRNA's coding for thyroid hormone receptors and 2 mRNAs arising from these genes which do not code for thyroid binding proteins has posed challenging functional questions. Efforts are under way to define the basis of hormonal specificity, the interaction of the receptors with other nuclear proteins and cis-acting regions of target genes, and the mechanism by which thyroid hormone regulates gene expression. These developments have also presented tools for solving important physiological questions. Thus, it may be possible to identify specific genes as direct thyroid hormone targets and to trace at the molecular level the circuits which lead to the recognized metabolic and morphogenic effects of the hormone. The objective of the conference is the rapid dissemination of the ongoing research in the hope that interlocking data will surface which will lead to new concepts and productive lines of investigation.